Minor changes
Added comments to components in DC examples + fixed few models after changing the cable model
Major revision: changes the record that contains the cable properties and simplified the base class that describe the cable. Now it contains just a record that describe the cable and it can either be a low or medium voltage one (replaceable). The function that compute the properties of teh cables have been moved directly in the records. Added documentation, redefined records for cables, added examples that show how to redeclare cables using medium voltage commercial cables, added examples to unit test.
Redefined reference results after found bug in sensor model